Omni-directional dielectric lens reflector and method of manufacturing same



LXE IRBE! //7 Jan. 30, 1968 NORIOMI OCHIAI 3, 66,965

OMNI-DIRECTIONAL DIELECTRIC LENS REFLECTOR AND METHOD OF MANUFACTURINGSAME Filed Dec. 10, 1964 2 Sheets-Sheet 1 INVENTQR. A arvam/ OCA/a/ zanJan. 30, 1968 Filed Dec. 10, 1964 9P 1101102; s90}? may: v

NORIOMI OCHIAI 3,366,965

OMNI-DIRECTIONAL DIELECTRIC LENS REFLECTOR AND METHOD OF MANUFACTURINGSAME 2 Sheets-Sheet 2 0216; Theoretical vaLue 0 I201401601802002403102602803flfl32034036 5; Azimuth INVENTOR.

War/om Oclwa/ M! TTORNEYS United States Patent 3,366,965OMNI-DIRE'CTIONAL DIELECTRIC LENS REFLECTOR AND METHOD OF MANU-FACTURING SAME Noriomi Ochiai, Suginami-ku, Tokyo, Japan, asa'gnor toKabushikikaisha Tokyo Keiki Seizosho (Tokyo Keiki eizosho Co., Ltd.),Tokyo, Japan, a corporation of apan . Filed Dec. 10, 1964, Ser. No.417,373 Claims priority, application Japan, Dec. 13, 1963, 38/67,171 7Claims. (CL 343-911) ABSTRACT OF THE DISCLOSURE An omni-directional lensreflector consisting of a spherical core, an outer shell and anintermediate shell interposed between the spherical core and the outershell, the respective dielectric constants K of the spherical core, theintermediate shell and the outer shell being selected in accordance withthe equation where r represents a normalized radius.

This invention relates to an omni-directional dielectric lens reflector,more particularly to a highly efficient omnidirectional reflector formicrowave use which could not have been obtained in the past and to amethod of the manufacture of the same.

The present invention principally lies in the provision of anomni-directional dielectric lens reflector in which waves having enteredit upon impingement of microwaves on a spherical lens thereof arereflected back in the direction of the incident waves along an ellipticpath in accordance with dielectric constants of a dielectric materialforming the lens.

An object of the present invention is to provide an omnidirectionaldielectric lens reflector which could not have been put to practical usein the past and the efliciency of which is enhanced by minimizing lossin the lens.

Another object of the present invention is to provide a method ofmanufacturing an omni-directional dielectric lens reflector, accordingto which gaps between adjacent concentric spherical layers are filled upso as to remove internal loss between the layers due to the gaps,thereby enhancing the efficiency of the omni-directional dielectric lensreflector.

Another object of the present invention is to provide a method ofmanufacturing an omni-directional dielectric lens reflector, accordingto which thickness of concentric spherical layers and their dielectricconstants can readily be made uniform so that the omni-directionaldielectric lens reflector can show its excellent ability.

Other objects, features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings, in which:

FIGURE 1 is a diagram, for explaining operations of an omni-directionaldielectric lens reflector;

FIGURES 2A to 20 are perspective views of dielectric hemisphericalshells, for explaining a prior art method of the manufacture of anomni-directional dielectric lens reflector which has been studied bysome people;

FIGURE 3 is a graph illustrating variations in the specific dielectricconstant of a dielectric material of an omni-directional dielectric lensreflector of this invention in response to the blending of finematerials;

FIGURE 4 is a diagram illustrating an example of a method of manufactureaccording to this invention;

FIGURE 5 is a perspective view of a hemispherical portion of theomni-directional dielectric lens reflector assembled according to themethod of this invention;

FIGURES 6A and 6B are diagrams for illustrating another method of thisinvention; and

FIGURE 7 is a characteristic curve illustrating an example ofmeasurements of the omni-directional dielectric lens reflectorconstructed according to this invention.

Referring to the drawing, the present invention will hereinafter beexplained.

With an attachment of a radar reflector to a small target whosereflecting power is low, its detection by a radar can be made easier.For this purpose a corner reflector or a Luneberg lens reflector hasheretofore been employed, but they do not act etfectively in alldirections. Reflectors having an omni-directional reflectioncharacteristic are much desired for various uses. However, there havenot ever been realized such desirable reflectors which satisfy therequirement. This invention has been devised to provide a reflectorwhich is capable of fulfiling the requirement of an excellentomni-directional reflection characteristic. This omni-directionaldielectric lens reflector consists of a spherical dielectric 1 asillustrated in FIGURE 1 and its specific dielectric constant K is afunction of a distance r alone from the center thereof and can beexpressed as follows,

where r is a normalized radius and K is infinite at the center of thesphere and 1 at the outer periphery. It is well-known that the aboveformula can be derived by putting one focus to be +00 and the other oneto be +00 similarly in the general solution by Luneberg. If now a planewave 2 strikes the lens 1 as illustrated in FIGURE 1, the wave traversesan elliptic path back in the direction from which it came. The structureof the lens is symmetrical with respect to the center and hence the lensacts as an omni-directional reflector. As is seen from the foregoingequation, the specific dielectric constant of the omni-directionalreflector is l at the outer periphery thereof and infinite at thecenter. However, a dielectric material of low loss and infinite specificdielectric constant cannot be obtained at present. Instead of such anunobtainable dielectric material, a dielectric one whose specificdielectric constant is up to about may well be employed, if some losses(about 10%) are sacrificed in practical use. The dielectric lens can beproduced by hollowing out the center thereof less than about 2.5% inradius or filling up the space with the same material. It has heretoforebeen impossible, however, to make uniformly dielectrics of low loss andhigh dielectric constant as predetermined, so that no omni-directionaldielectric lens reflector has been put to practical use in anycountries.

It is diflicult in practice to make a dielectric lens reflector thespecific dielectric constant of which varies continuously from 1 toabout 100 as a function of the radius. Accordingly, a method is adoptedsuch that the specific dielectric constant is varied in a stepwisemanner by dividing a lens reflector into 10 to 50 concentric sphericalshells in accordance with the dimension of its diameter.

To cite an example of such conventional method, radiuses and specificdielectric constants of respective layers in an omni-directionaldielectric lens reflector 128 mm, in diameter are given in the followingtable.

This reflector was designed for use at frequencies lower than the X band(a region of a wavelength of 3 cm.) and the thickness of the layers mustbe selected less than t/4 /K ()tzwavelength). Where the thickness ismore than )\/2 /K, waves having entered the lens causes a trappingphenomenon, from which loss results. A method of manufacture which hasbeen attempted by some people is a method described later such thattitanium dioxide or metallic flakes are mixed with polystyrene foambeads or polyethylene foam material and the mixture is heated to bemolded by foaming into hemispherical shells t/4 /K in thickness). Theirspecific dielectric constants are adjusted in accordance with thecompounding ratio of the materials. This method is referred to as a heatfusion method. In the conventional heat fusion method, however, it isdifficult to make distribution of the specific dielectric constantuniform in the same layer. Particularly in the case of omni-directionaldielectric lens reflectors requiring high specific dielectric constants,it becomes more and more difficult to produce dielectric materials ofuniform specific dielectric constant distribution, since the layers arethin due to )t/4 /K and those below the fourth layer are as thin as 3cm., as is apparent from the foregoing table.

That is, in the conventional method, for example, polystyrene beads arepacked into a spherical closed mold and heated at a suitabletemperature, for instance, 120 C., for several minutes to foam to bemolded, into a spherical shell. A plurality of thus molded shells areassembled one on another into a spherical omni-directional dielectriclens reflector, if necessary, by means of a binder. In order to obtain ashell of high dielectric constant, a mixture of polystyrene beads andgrains of titanium dioxide is employed. However, the inventor of thepresent invention has found that such a heat fusion method heretoforeused possesses disadvantages described hereinbelow. That is, this methodemploys a metal mold consisting of two parts which are provided withadditional equipments such as an inlet for the materials, confrontingmarginal edges, handles and so on. Therefore, the metal mold is notusually uniform in thickness over the entire area thereof. Beads of theaforementioned foam material are packed into such a mold and placed in achamber. Then, the mold is heated by applying, for example, heatedsteam, into the chamber. However, since the thickness of the respectiveportions of the mold is not uniform as described above, the relativelythinner portion of the mold transmits heat to the inner part thereofmore rapidly than the relatively thicker portion. The foam material atthe thinner portion begins to foam more rapidly than at the thickerportion, so that the foam material at the latter portion is compressed.Accordingly, the density of the foam material at the thicker portionbecomes large and its dielectric constant increases. Furthermore, whenthe heated steam is removed and the mold is cooled the foam material incontact with the inside of the metal mold is rapidly cooled. However,since the molding is a bad conductor of heat, the foam material remotefrom the inside of the mold still continues to foam to some extent andits mass tends to be less dense than that of the former. For thesereasons, the dielectric constant distribution which is required to beuniform varies about :10% in the same spherical shell according to theconventional heat fusion method, and accordingly it is diflicult toobtain a highly efficient dielectric lens by assembling such sphericalshells.

Furthermore, according to the conventional heat molding by foamingdescribed above, titanium dioxide or metallic flakes, which are suitablefor obtaining layers each having a desired dielectric constant, aremixed with the aforementioned foam materials and the mixture is heatedto be molded by foaming into hemispherical shells corresponding to therespective layers mentioned in the foregoing. Then, the resultant shellsare assembled spherical about a core. In order to make possible theassembly of the respective shells, tolerance of the radius of the moldedshells is required to be $0.25 mm. Accordingly, it is naturallyconsidered that where the shells of such tolerance are assembled gaps of0.5 mm. at maximum are made between the adjacent layers. In presence ofsuch gaps between the layers of high dielectric constant, internalreflections occur between the layers to cause propagation loss, sincethe dielectric constant of the gap is about 1. The hemispherical shellsof the respective layers thus made will hereinbelow be explained moreconcretely with reference to FIGURE 2. Dielectric hemispherical shells 4and 5 of the inner diameter R and R and outer diameters R and Rrespectively, obtained by the heat molding as described above, areassembled one on the other to be a hemispherical shell 6 of the innerdiameter R and outer diameter R Since the layers are thus assembled in asequential order from the inner ones to be a sphere, there are gaps of,for example, 0.5 mm. at maximum between the adjecent layers as describedabove. Accordingly, the conventional method like this has a disadvantagethat efliciency of the dielectric lens is caused to lower due tointernal reflections between the adjacent layers.

This invention is to provide an omni-directional dielectric lensreflector in which distribution of a specific dielectric constant isuniform in the same layer and no gap exists between the adjacent layersthereby to eliminate loss due to the gap. In the present invention,polystyrene beads are preheated at a suitable temperature lower than 70C. to effect partial foaming thereof, by which the beads are suitablyexpanded so as to obtain a predetermined dielectric constant. Foradjusting the dielectric constant, the grain size of the heads isselected beforehand. That is, grains or powders of a desired grain sizeare chosen and, if necessary, titanium dioxide is added thereto. Then, abinder is mixed into the material thus selected. An inner layer havingalready hardened is surrounded by such mixed material. In this case, itis a matter of course to use a suitable mold in order that the outerlayer may be concentric with the inner layer and its respective partsmay be uniform in thickness. However, heating is not effected such thatthe beads may foam again. If heating is made, its heating temperature isheld merely enough to evaporate the solvent in the binder. In somecases, it is possible to evaporate the solvent without heating. Thus,the dielectric constant distribution in the same layer can be madeextremely uniform and gaps between the adjacent layers can beeliminated. The present invention will hereinafter be explained indetail.

In short, the present invention essentially resides in the provision ofan omni-directional dielectric lens reflector which comprises aspherical core composed of a dielectric material, an outer shell made ofa dielectric material, surrounding the spherical core substantiallyconcentrically therewith, and an intermediate shell composed of adielectric material and disposed between the spherical core and theouter shell. Each of the respective portions has a specific dielectricconstant according to the equation of TABLE II 1st Iayer-e=83.4

Material: Mixed weight, g. 7 5 z 77.4 where r is a normalized radius andK is a specific dielec- Galione clay tric constant which is infinite atthe center and 1 at the Cacoe outer periphery of the lens. The sphericalcore consists of li one block produced by ceramic dielectrics of highdielecz 0-45 tric constant and relatively low microwave loss containingtitanium dioxide. The intermediate shell consists of a plu- 10 Bakmgtemperature: 1280a -r 3 rality of concentric spherical layers and atleast one inner TABLE III portion thereof is a hom ogeneous layer suchthat fine 2nd laye, =43.7 gram or powder of high dielectric constant ismixed with Percent b h y wer t a binder and bound thereby. The outershell consists of Ticon M a 60 one or a plurality of foamed plasticlayers. The relation- Ticon C 40 ship of the specific dielectricconstants of the respective Baking temperature: 3500 C 3 hr. layers 15such as to satisfy the foregoing equation, and I further the sphericalcore and almost all the layers of the M and C: mammals by TAMintermediate shell adhere closely to one another without In this examplethe core Consists of two y but the gaps therebetween. core can becomposed of one block by using ceramic di- The method of the manufactureof an omni-directional t i sdielectric lens reflector of this inventionwill hereinafter be The intermediate Shell comprises third to twelfthlayersexplained with reference to Table I. A material of the third andfourth layers can be of com- This is an example of the manufacture of adielectric Position Such, for p as Shown in FIGURE lens reflector inwhich a spherical core is composed of 11 FIGURE the ordinates representthe dielectric two block an intermediate hell is composed of t nconstantKof the mixture of granular and powdered titanispherical layersand an outer shell consists of three spherum dioxide with a binder, andthe abscissa the quantity 0 ical layers. In the Table I the Column Irepresents the layof powdered titanium dioxide in grams per cubic inchof er number, the Column II the specific dielectric constant granulartitanium dioxide, the granular material comand permissible deviation,the Column III the radius and prising grains of 0.4 mm. in diameter andof the bulk the Column IV the compound weight of grain or powder density1.89. The values of the blending for the third and of titanium dioxide,both of which are finely divided ones fourth layers shown in the Table Iare examples obtained obtained by grinding baked ceramic dielectrics.The Colfrom the curve in FIGURE 3. In order to join the third umn Vshows the weight of pre-expanded polystyrene layer to the second layer,a uniform mixture of the aforefoam beads, the degree of expansion beingdetermined by mentioned titanium dioxide grain, powder and polyvinyl thebulk density (bd) in grams per cubic centimeter. The acetate emulsion isput spherically on the core concen- Column VI shows weight of polyvinylacetate emulsion. trically therewithand uniform in thickness over theentire TABLE I I II III IV V VI Specific Grain of Powder of PartiallyEx- Polyvinyl Layer Number Dielectric Radius TlOz TIO; (gt) panded Poly-Acetate Constant (mm.) d=(()-; r)nm styretsgieads lgtnlsion SphericalCore:

eaeeassaee anese 4. 75 0 0. 23 0 0. 0.45 0 1. 30 0. 51 O 4. 9O 1. I7 323:5 51 333 0 35. 30 4. 34 0 57. 20 6. 54 0 78. 20 7.89

0 86.00 bd=0.48 10.50 0 84.00 bd=0.31 15.50 0 46. 00 bd=0. 12 0 Sincethe dielectric constant of the core designed as the first and secondlayers in the present example exceeds 40, the core is made of ceramicdielectrics that a mixture of various materials are baked, so as to keepthe dielectric constant at a predetermined value. A formation of thecore is carried out as follows. That is, materials given in the Table IIare first baked to obtain a spherical core of one block and a sphericallayer made of a mixture of materials stated in the Table III is put onthe outside of the spherical core and then it is baked to produce asecond layer. It must be noted that the first and second layers are inclose contact with each other without any gaps therebetween according tothe present method.

area. The second and third layers are stuck together due to the binderwith practically no gap therebetween. The fourth layer is likewise stuckclosely to the third layer without any gaps therebetween.

An example of materials of the fifth to tenth layers is as given in theTable I. As is apparent from the Table I, a mixture is employed which iscomposed of grains or titanium dioxide, and grains produced by heatfoaming of polystyrene foam beads and polyvinyl acetate emulsion.

In this case, when the fifth layer is formed on the fourth layer, auniform mixture of the aforementioned three materials is laid on thefourth layer concentrically and uniform in thickness and stuck to thefourth layer due to evaporation of the solvent in the binder with orwithout heating in a state that the grain of polystyrene beads does notfoam again. Accordingly, the fifth layer is one stratiform block andadheres closely to the fourth layer without gaps therebetween.

A sixth layer is stuck closely to the fifth layer in the same manner asthe fifth layer to the fourth one. Thus, the layers up to a twelfth oneare thus assembled one after another. Therefore, it is apparent that theintermediate shell from the fifth to the twelfth layers adhere closelyto adjacent layers without gaps therebetween.

In the present example, the outer shell consists of thirteenth tofifteenth layers and the thirteenth and fourteenth layers are thosewhich foamed polystyrene beads are bound by a polyvinyl acetateemulsion. In this case, a bulk density of foamed polystyrene heads isdifferent from those of the aforementioned inner layers and as given inthe Table I. The thirteenth layer is also stuck to the twelfth layer inthe same manner as the inner layers described above. The layers of theouter shell are joined to the inner layers one after another withoutgaps therebetween in a state that foamed polystyrene beads do not foamagain. The fifteenth layer integrally encloses the aforementionedfourteenth layer with two hemispherical shells, each produced by heatmolding of pre-foamed polystyrene grains packed in a predeterminedhemispherical mold. In this case, this final layer has a dielectricconstant approximately equal to that of air, and hence even if there isa little gap between the fourteenth and fifteenth layers, such gap canbe neglected in practice.

In the foregoing example, the dielectric constants of the respectivelayers are determined as given in the Table I and it goes without sayingthat the dielectric constants satisfy the aforementioned equation.

In the above example, it is possible to obtain dielectric constantssubstantially equal to the aforementioned ones by suitably adding thematerial in the Column V of the Table I in case of changing thematerials in the Column IV for the third and fourth layers. Also in thefifth layer and several layers outside of it, specific dielectricconstants similar to those given in the Table I can be obtained byintroducing titanium dioxide powder into the materials in the Column IVand increasing or decreasing (including zero) the amount of the grain offoamed polystyrene beads. Furthermore, it is also possible in the layersof the outer shell that even if the grain of foamed polystyrene beads oflower bulk density is used, similar specific dielectric constants can beobtained by introducing grain or powder of titanium dioxide. In thiscase, the layers of the outer shell can be regarded as those of theintermediate shell and further this outer shell can be formed in asingle layer.

Referring now to the drawing, an embodiment of the present inventionwill hereinafter be explained.

FIGURE 4 illustrates an example in which after first and second layers 9and 10 described in the foreging example have been assembled sphericallyhemispherical outer layers are joined to it one after another. In thefigure the spherical core composed of the first and second layers 9 and10 is placed in a mold 8, and the core is positioned correctly in thecenter of the mold with a uniform gap between its outer periphery andthe mold. Materials for a third layer 11 are packed into the gap, andthe third layer is formed as in the foregoing example. Thus, the thirdlayer is formed in close contact with the second layer. As a result ofthis, the present method is advantageous in removal of gaps betweenadjacent layers inevitably experienced in the prior art method. In thisprocess, when the aforementioned binder has hardened a resultantassembly is removed from the mold. It is then placed in a mold for thenext layer to form that layer similarly. Thus, many layers are moldedone after another. After all the layers have been molded, such twohemispheres are secured to each other to be spherical, obtaining anomnidirectional dielectric lens reflector. In order to prevent thematerial 8 from adhering to the mold, it is possible that a sheet of,for example, synthetic resin such as a vinyl sheet is spread on theinside of the mold and removed therefrom after the layer is molded andremoved from the mold.

It has been found that, in the present method, distribution of aspecific dielectric constant can be made extremely uniform in the samelayer by well stirring mixed materials to be uniform in the mixture.Such uniform distribution of the specific dielectric constant could nothave been realized by the conventional heat fusion method. In view ofresults obtained, it is easy in the present invention that precision ofthe specific dielectric constant is made as given in the Table I and ithas been made possible for the first time by the present method toprecisely control the precision of the specific dielectric constant inpractical use.

FIGURE 5 is a sketch of the hemisphere described in connection withFIGURE 4. In an actual manufacture all the layers are not required to bestuck closely to one another. Since the ratio of permissible deviationsof the specific dielectric constant in an nth layer and an n-l layer ofthe outer shell is relatively great, no troubles are caused inperformance, even if layers according to the conventional heat fusionmethod are employed as the layers of the outer shell. In this case, suchprior art layers are preferable from the strength standpoint. In somecases, some of the intermediate layers can be molded by the heat fusionmethod.

In the foregoing, two hemispherical members are assembled to be asphere, but the members are not always required to be formedhemispherically. That is, the sphere can be assembled by four memberseach being a quarter of the sphere or by a member of a quarter and amember of three quarters. The sphere can also be obtained by assemblinga plurality of members of such shape that the resultant assembly becomesspherical. Where a sphere is made up by assembling partially sphericalmembers according to this method, it is necessary to consider mechanicalprecision so that the respective layers may not be caused to bediscontinuous at their junction.

A method of the manufacture will hereinbelow be explained which does notrequire such consideration.

A spherical core composed of first and second layers is made in the samemanner as in the foregoing example. Then, a material is prepared forother layers. That is, foamed polystyrene beads, grain of titaniumdioxide (0.4 mm. in diameter), titanium dioxide powder, polyvinylacetate emulsion, are suitably compounded in such a manner as to obtaina predetermined specific dielectric constant. Using the above materials,spherical layers are molded one after another as in the foregoingexample.

In FIGURE 6, a second layer 20 is placed as a core in a mold 21, and onehalf of the sphere is positioned correctly in the center of the moldwith a uniform gap between its outer periphery and the mold. A materialfor a third layer is then packed into the gap and molded. After this, amaterial 23 for other half of the third layer is tamped into a mold 22and pressed on the other half of the second layer and molded. Then, thethird layer is molded spherically in close contact with the secondlayer. Accordingly, the present method is advantageous in that there aremade no gaps between the adjacent layers nor junction planes between thetwo hemispheres. When the binder has hardened the sphere is removed fromthe mold and then placed in a mold of the next layer. Thus, sphericallayers are likewise molded one after another. By molding all the layersin this manner, an omni-directional dielectric lens reflector iscompleted. In this example when outer layers are molded on the outerperiphery of the spherical core one by one, layers of a hemisphericalportion are molded at first and then layers of the other hemisphericalportion are molded, thereby to form spherical layers. However, it is notalways necessary to form the layers hemispherically. It is also possibleto complete a spherical layer by firstly molding a partially spherical 9portion and then the other portion. Furthermore, a spherical mold can beemployed. That is, a molded core is positioned in the center of thespherical mold with a predetermined uniform gap between the core and themold, and a material for an outer layer is tamped into the gap andmolded.

In order to prevent the material from adhering to the mold, it ispossible in this case that a sheet of, for example, synthetic resin suchas a vinyl sheet is spread on the inside of the mold and removedtherefrom after the layer has been molded and removed from the mold.

Also in this method, distribution of the specific dielectric constantcan be made extremely uniform by sufficiently stirring the mixedmaterial to be uniform in the mixture prior to packing the material intothe gap between the mold and the previously molded sphere. Such uniformdistribution is impossible to obtain by the conventional heat fusionmethod. Also in the present example, precision of the specificdielectric constant may easily be made as given in the foregoing table.With this method, an excellent omni-directional dielectric lensreflected can be put to practical use for the first time.

In actual practice of this method all the layers need not be molded asin the foregoing method. Since one or two layers of the outer shell haverelatively large ratios of permissible deviation of the specificdielectric constant, layers made by the conventional heat fusion methodcan be employed with practically no trouble in actual use. From thestrength standpoint, the heat fusion method is preferred in some cases,so that layers by this conventional method can be used as one or twolayers of the outer shell. In addition, a lens may also be manufacturedby molding some of the layers of intermediate shell according to theheat fusion method.

It must be noted in the above described method of manufacturing theomni-directional dielectric lens reflector that the outer layer of theintermediate shell and the inner layer of the outer shell respectivelyadhere to the adjacent inner layer thereof including the spherical corewith the mixture itself of the outer layer when the solvent of thebinder is evaporated from the mixture.

FIGURE 7 illustrates an example of measured results by an X band of theomni-directional dielectric lens reflector fabricated according to thismethod. This refiector is 128 mm. in diameter and composed of fifteenlayers, which was made on the basis of the values given in the Table I.In FIGURE 7, the ordinate expresses measured radar cross-section in dband the abscissa shows azimuth, db being atheoretical value of radarcrosssection.

Two way loss of electromagnetic waves is about 5.4 db and the reflectioncharacteristic varies only withinilA db in accordance with direction. Inthe conventional Luneberg lens the two way loss of electromagnetic wavesis within 2.0 db and, in order to give a radar cross-section equal to amaximum radar cross-section of the Luneberg lens, an omni-directionaldielectric lens reflector which is 20% larger in diameter may well beused. Thus, the reflector has a merit that an omni-directionalreflection characteristic can be obtained together with a value equal tothe maximum radar cross-section of the Luneberg lens reflector. In caseof using the Luneberg lens as having the same efficiency as theomni-directional dielectric lens reflector, at least eightsimilar-shaped Luneberg lenses using specific reflecting metallic platesare required to be arranged suitably in actual use. Accordingly, it willbe seen that the omni-directional dielectric lens reflector of thepresent invention is extremely excellent.

It will be apparent that many modifications and variations may beefl'ected without departing from the scope of the novel concept of thisinvention.

What is claimed is:

1. An omni-directional dielectric lens reflector comprising a sphericalcore composed of a dielectric material, an outer shell composed of adielectric material and I0 surrounding said spherical core substantiallyconcentrically therewith, and an intermediate shell composed of adielectric material and interposed between said spherical core and saidouter shell, each of said respective portions of said spherical core,outer shell and intermediate shell having a specific dielectric constantof r being a normalized radius, said spherical core consisting of ablock made of ceramic dielectrics of relatively low microwave losscontaining titanium dioxide, said intermediate shell consisting of aplurality of concentric spherical layers, at least one inner layer ofsaid plurality of concentric spherical layers being homogeneous andcontaining high dielectric constant material, said outer shellconsisting of at least one layer composed of foamed I plastics, thespecific dielectric constant of said respective layers satisfying saidequation, and said spherical core and almost all the layers of saidintermediate shell adhering closely to one another without gapstherebetween.

2. An omni-directional dielectric lens reflector as claimed in claim 1,wherein one block of said spherical core consists of a plurality oflayers.

3. An omnidirectional dielectric lens reflector as claimed in claim 1,wherein the inner portion of said intermediate shell is a solidhomogeneous mixture portion of grains and powders of titanium dioxideand polyvinyl acetate and the outer portion thereof is a solidhomogeneous mixture portion of grains of titanium dioxide, expandedpolystyrene beads and polyvinyl acetate.

4. An omni-directional dielectric lens reflector as claimed in claim 1,wherein said intermediate shell consists of a solid homogeneous mixtureof metallic particles, expanded plastic beads and polyvinyl acetate.

5. An omni-directional dielectric lens reflector as claimed in claim 1,wherein said outer shell consists of a solid homogeneous mixture portionof expanded plastic beads and polyvinyl acetate.

6. A method of manufacturing an omni-directional dielectric lensreflector which comprises a spherical core composed of a dielectricmaterial, an outer shell composed of a dielectric material andsurrounding said spherical core substantially concentrically therewithand an intermediate shell composed of a dielectric material andinterposed between said spherical core and outer shell, said respectiveportions each having a specific dielectric constant of r being anormalized radius, comprising the steps of preparing said spherical coremade of ceramic dielectrics of high dielectric constant and lowmicrowave loss containing titanium dioxide, surrounding concentricallysaid spherical core with a first layer of said intermediate shell, saidfirst layer being a homogeneous layer of a mixture of fine grains orpowders of a high-dielectric-constant material with a binder,evaporating the solvent of said binder so that a homogeneous layer thedielectric constant of which satisfies said equation adheres over saidspherical core with said mixture itself without any gaps therebetween,thereafter surrounding concentrically said homogeneous layer with asecond layer of said intermediate shell, said second layer being of ahomogeneous layer of another mixture of fine grains or powder ofhigh-dielectricconstant material with a binder, evaporating solvent ofsaid binder so that another homogeneous layer the dielectric constant ofwhich satisfies said equation adheres over said first mentionedhomogeneous layer with said second mentioned mixture itself without anygaps therebetween, thus forming other layers of said intermediate shellone after another in the same way, then surrounding concentrically theoutermost layer of said intermediate shell with a mixture of foamedplastic grains with a binder 11 and evaporating solvent of said binderso that a homogeneous layer of said outer shell the dielectric constantof which satisfies said equation adheres over said outermost layer ofsaid intermediate shell with said mixture itself without any gapstherebetween.

7. A method of manufacturing an omni-directional dielectric lensreflector, as claimed in claim 6, comprising the steps of forming thefirst layer of said intermediate shell over said spherical corehemispherically, forming the second layer over said first layerhemispherically, forming similarly other layers of said intermediateshell and layers of the outer shell of predetermined numbers over theinner layers one after another in the form of a hemisphere, preparinganother hemisphere similar to the above one except said spherical core,and assembling said two hemispheres into a lens sphere.

References Cited UNITED STATES PATENTS 2,835,891 5/1958 Peeler et a1.343-911 2,943,358 7/1960 Hutchins eta] 343-911 OTHER REFERENCES Peeleret al., Microwave Stepped-Index Luneberg Lens, April 1958, 6 pages.

ELI LIEBERMAN, Primary Examiner.

